It is narrated from Sayyiduna Wathilah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever falsely accuses a dhimmi (a non-Muslim under Muslim protection) will be given the prescribed punishment for it on the Day of Resurrection with whips of fire." (The narrator said:) I asked Makhul: What is the most severe thing that may be said? He said: To be told: "O son of a disbeliever!"
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18355